As communication demands have been ever increasing, the high level communication protocols have been implemented by transmission apparatuses and control information transmitted and received between apparatuses has been increased. In a synchronous transmission network, such as a synchronous optical network (SONET)/synchronous digital hierarchy (SDH) network and the like, an overhead area that accommodates control information is provided in a frame. For example, a SONET/SDH frame has a hierarchy structure and areas that accommodate various types of control information (for example, K1, K2, C2, D4 bytes) are fixedly provided therein.
On the other hand, in Ethernet (registered trademark), which is an asynchronous transmission method, an area that accommodates such control information is not defined in an Ethernet frame in accordance with the specification. Therefore, in Ethernet, in addition to an Ethernet frame (which will be hereinafter referred to as a “user frame”) that accommodates user data serving as a main signal, an Ethernet frame (which will be hereinafter referred to as a “control frame”) that accommodates control information of communication protocols and the like is used.
Regarding packet transmission technology, such as Ethernet, for example, International Publication Pamphlet No. WO 2003/017577 describes that the transmission interval of a priority packet is reduced to be shorter than the generation interval thereof to thereby provide a margin period and a non-priority packet is transmitted in the margin period. Japanese Laid-open Patent Publication No. 5-268256 describes that, when an asynchronous transfer mode (ATM) cell has a margin area of 3 bytes or more, congestion information for an ATM exchanger is stored in the margin area. Japanese Laid-open Patent Publication No. 2009-153028 describes that a start frame delimiter (SFD) is extended to identify whether a network layer 2 frame is user data or maintenance management data.
Regarding SDH transmission technology, Japanese Laid-open Patent Publication No. 8-172485 describes that a first monitor pattern is inserted to a free time slot in a fixed location in section management information and a second monitor pattern is inserted in a free area in path management information.